Medical image processing apparatus and method

ABSTRACT

A medical image processing apparatus comprises processing circuitry configured to: obtain a three-dimensional (3D) image that is representative of an anatomical region of a subject; acquire a stream of two-dimensional images that are representative of the anatomical region of the subject; set a first rendering direction by performing a 2D/3D registration procedure in respect of the 3D image and a 2D image of the stream of 2D images; generate a first rendered image from the 3D image based on the first rendering direction; and for each of a plurality of subsequent 2D images of the stream of 2D images, determine whether a condition is satisfied, the condition being dependent on at least one of a 2D misalignment and a time since last 2D/3D registration procedure; when the registration condition is satisfied, select one of the subsequent 2D images; reset the rendering direction to obtain a second rendering direction by performing a 2D/3D registration procedure in respect of the 3D image and the selected 2D image; and regenerate a second rendered image from the 3D image based on the second rendering direction.

FIELD

Embodiments relate generally to medical imaging in which a 3D scan is registered with live 2D images, for example in fluoroscopy-based live guidance of needle or catheter interventions.

BACKGROUND

It is known to acquire volumetric imaging data that is representative of an anatomical region of a patient or other subject. Volumetric imaging data may be acquired in advance of performing a medical procedure on that anatomical region. Volumetric imaging data that is acquired in advance of a procedure may be referred to as pre-operative volumetric data.

Pre-operative volumetric data may be obtained using any suitable three-dimensional imaging modality, for example computed tomography (CT), magnetic resonance (MR) or cone-beam CT (CBCT).

In the discussion below, a set of volumetric image data may also be referred to as an image volume, a three-dimensional (3D) image, or a 3D scan.

It is known to acquire live 2D imaging data while a medical procedure is being performed. The live 2D imaging data may comprise a stream of 2D images that are acquired in real time or near-real time.

Live 2D imaging that is performed during a medical procedure may also be referred to as intra-operative 2D imaging or real-time 2D imaging.

Intra-operative 2D images may be obtained using any suitable 2D imaging modality. For example, 2D fluoroscopy images may be obtained using an X-ray scanner, or 2D ultrasound images may be obtained using an ultrasound scanner.

Intra-operative 2D images such as fluoroscopy may be used for live guidance of medical devices towards (or away from) specific anatomical targets. A clinician may use the stream of live 2D images to determine a current position of a medical device within a patient's body, and to navigate the medical device through the patient's body in real time. For example, the medical device may be a needle or a catheter. A specific anatomical target may be any anatomical target that is visible in the 2D data, for example a tumor, bone, lung, or blood vessel.

Methods are known in which both a pre-operative 3D image and a stream of 2D intra-operative images are presented to a user, for example a clinician. The pre-operative 3D image may assist the clinician in guiding the medical device to (or away from) an anatomical target. The anatomical target may be any anatomical target that is visible in both the 2D and the 3D data, for example a tumor, bone, lung, or blood vessel.

The 3D data may provide information that is not available from the 2D data alone. The presence of a third dimension may provide the clinician with additional navigational information. In some circumstances, the 3D data may be of higher resolution than the 2D data. In some circumstances, the 3D data may be acquired using a different imaging modality from the imaging modality used to acquire the 2D data. The use of the different imaging modality may provide additional information to the clinician.

It may be considered that a frontier in image-guided intervention is the reliable alignment of pre-operative volumetric data with intra-operative 2D images.

To provide the user (for example, clinician) with useful information, a coordinate system of the volumetric data should be aligned with a coordinate system of the intra-operative 2D images.

Once the coordinate systems are aligned, images may be presented such that the position of anatomical features in an image rendered from the 3D data is the same as the position of those anatomical features in the 2D images. A fusion image may be is obtained by fusing data from a 2D image and 3D image.

To align a 2D image and a 3D image, a 2D/3D registration process may be performed. 2D/3D registration may be time-consuming and/or computationally complex.

In some forms of 2D/3D registration, a Digitally Reconstructed Radiograph (DRR) is rendered from the 3D image. A DRR is a type of 2D projection of a 3D scan. The DRR is then registered to the 2D image.

Rendering a DRR may be time intensive. A single 2D/3D registration process may comprise multiple iterations of obtaining a DRR. The use of multiple DRR iterations may increase the time taken for the 2D/3D registration.

A DRR may have different image properties (for example, intensity and contrast) from a 2D image. Due to the different image properties, it may be the case that a multi-modality registration method (for example, mutual information) is used to register the DRR to the 2D image. In some circumstances, multi-modality registration methods may be more complex, more time-intensive and/or less accurate than methods for registering images of the same modality.

FIG. 1 shows an example of a fluoroscopy image of an anatomical region, which in the example of FIG. 1 is part of the spine. FIG. 2 shows a DRR that has been created from 3D data that is representative of the same anatomical region. The DRR is scaled to be of a similar scale and magnitude as the 2D data. However, it may be seen that the DRR of FIG. 2 and the fluoroscopy image of FIG. 1 do not look alike. In particular, the intensity values are very different.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are now described, by way of non-limiting example, and are illustrated in the following figures, in which:

FIG. 1 is a fluoroscopy image;

FIG. 2 is a Digitally Reconstructed Radiograph image that is created from 3D data, and is scaled to be of a similar scale and magnitude to the fluoroscopy image of FIG. 1;

FIG. 3 is a schematic diagram of an apparatus in accordance with an embodiment;

FIG. 4 is a schematic diagram of an X-ray diagnostic apparatus in accordance with an embodiment;

FIG. 5 is a flow chart illustrating in overview a 2D/3D registration method;

FIG. 6 is a flow chart illustrating in overview a method in accordance with an embodiment;

FIG. 7 is a schematic illustration of an induced rotation of a 2D image; and

FIG. 8 is a flow chart illustrating in overview a stream of frames and an asynchronous 2D/3D registration in accordance with an embodiment.

DETAILED DESCRIPTION

A medical image processing apparatus 10 according to an embodiment is illustrated schematically in FIG. 3.

The apparatus 10 comprises a computing apparatus 12, in this case a personal computer (PC) or workstation, which is connected to a computed tomography (CT) scanner 14, an X-ray scanner 15, one or more display screens 16 and an input device or devices 18, such as a computer keyboard, mouse or trackball.

The CT scanner 14 is configured to obtain volumetric CT data that is representative of an anatomical region of a patient or other subject. In the present embodiment, the volumetric CT data is acquired before a procedure is performed, i.e. pre-operatively. The volumetric image data may also be referred to as a volumetric image or 3D image. Operations described as being performed on the volumetric image or 3D image may be performed on a 3D data set comprising a plurality of voxels and corresponding voxel values (for example, voxel intensity values).

In alternative embodiments, the CT scanner 14 may be replaced or supplemented by a scanner configured to obtain volumetric imaging data in any appropriate imaging modality, for example a CT scanner, cone-beam CT scanner, MRI (magnetic resonance imaging) scanner or ultrasound scanner. In some embodiments, the volumetric imaging data may be obtained by acquiring multiple two-dimensional scans.

The X-ray scanner 15 is configured to obtain X-ray data that is representative of the same anatomical region of the same patient or other subject. In the present embodiment, the X-ray data comprises 2D fluoroscopy data that is acquired during the performing of the procedure, i.e. intra-operatively. A set of 2D fluoroscopy data may also be referred to as an 2D image. Operations described as being performed on a 2D image may be performed on a 2D data set comprising a plurality of pixels and corresponding pixel values (for example, pixel intensity values).

In alternative embodiments, the X-ray scanner 15 may be replaced or supplemented by any scanner configured to obtain imaging data in any appropriate two-dimensional imaging modality, for example an ultrasound scanner configured to obtain live 2D ultrasound imaging, or a video camera configured to obtain live video images.

Imaging data sets obtained by the CT scanner 14 and/or X-ray scanner 15 may be stored in memory 20 and subsequently provided to computing apparatus 22, or may be provided to computing apparatus 22 directly. In an alternative embodiment, imaging data sets are supplied from a remote data store (not shown) which may form part of a Picture Archiving and Communication System (PACS). The memory 20 or remote data store may comprise any suitable form of memory storage.

In the present embodiment, the CT scanner 14 and X-ray scanner 15 are connected to the computing apparatus 12. Imaging data is provided by the X-ray scanner 15 in real time or near-real time. In other embodiments, the CT scanner 14 and/or X-ray scanner 15 may not be connected to the computing apparatus 12.

Computing apparatus 12 provides a processing resource for automatically or semi-automatically processing imaging data sets, and comprises a central processing unit (CPU) 22.

The computing apparatus 12 includes registration circuitry 24 configured to perform 2D/3D and 2D/2D registration processes, assessment circuitry 26 configured to determine whether a registration condition is satisfied, and rendering circuitry 28 configured to render images from 3D and/or 2D data.

In the present embodiment, the circuitries 24, 26, 28 are each implemented in computing apparatus 12 by means of a computer program having computer-readable instructions that are executable to perform the method of the embodiment. However, in other embodiments, the various circuitries may be implemented as one or more ASICs (application specific integrated circuits) or FPGAs (field programmable gate arrays).

The computing apparatus 12 also includes a hard drive and other components of a PC including RAM, ROM, a data bus, an operating system including various device drivers, and hardware devices including a graphics card. Such components are not shown in FIG. 3 for clarity.

An X-ray diagnostic apparatus 30 according to a further embodiment is illustrated schematically in FIG. 4. In the embodiment of FIG. 4, the computing apparatus 12 is integrated into an X-ray diagnostic apparatus 30, which may also be referred to as an X-ray scanner. The X-ray diagnostic apparatus 30 comprises an X-ray tube 32 configured to emit X-rays, and an X-ray detector 32 configured to detect X-rays emitted by the X-ray tube. For each frame of a stream of frames, the detected X-rays are used to form a 2D X-ray image which is supplied to the computing apparatus 12. The 2D X-ray images are intra-operative images as described above.

In the embodiment of FIG. 4, pre-operative 3D image data is stored in memory 20. In other embodiments, the pre-operative 3D image data may be obtained by the computing apparatus 12 from any suitable memory store or apparatus.

The apparatus of FIG. 3 is configured to perform methods illustrated in overview in the flow chart of FIG. 5 and the flow chart of FIG. 6, and illustrated schematically in FIG. 7. Similar methods may be performed using the apparatus of FIG. 4.

FIG. 5 is a flow chart illustrating in overview a 2D/3D registration process. The 2D/3D registration process is used to register pre-operative 3D data 40 with intra-operative 2D data 42. The intra-operative 2D data 42 comprises a 2D image acquired during a procedure.

In the discussion below with reference to FIG. 5, we consider only the 2D/3D registration of a single 2D image. In the subsequent discussion with reference to FIGS. 6 and 7, we consider registration of a multiple images of a stream of 2D images. In the embodiment of FIGS. 6 and 7, the 2D/3D registration process of FIG. 5 is performed for selected images of the stream of images, and is not performed for other images of the stream of images. By performing the 2D/3D registration on only selected images of a live stream of images (instead of on all images), a more efficient registration process may be obtained.

In the embodiment of FIG. 5, the pre-operative 3D data 40 comprises volumetric CT data. The pre-operative 3D data 40 may also be referred to as pre-interventional data 40.

The volumetric CT data 40 is acquired by the CT scanner 14 prior to the performing of a needle intervention procedure. In other embodiments, the volumetric data may be required prior to the performing of any form of intervention with fluoroscopy guidance, for example a catheter-based or probe-based intervention.

The volumetric CT data comprises a CT scan of the anatomical region, including a predetermined target location to which the needle is to be guided. The volumetric CT data 40 provides a detailed view of the anatomical region in three dimensions. The volumetric CT data 40 is representative of the anatomical region before the needle is introduced into the anatomical region.

The intra-operative 2D data comprises fluoroscopy data 42. The fluoroscopy data 40 comprises image data for a single frame of a live stream of fluoroscopy images acquired by the X-ray scanner 15 during the needle intervention procedure. The fluoroscopy image frame 42 is a live image of the anatomical region showing a current position of the needle within the anatomical region.

In other embodiments, the pre-interventional data 40 may be volumetric data of any suitable modality. The fluoroscopy data 42 may be any appropriate 2D data, for example a frame of a stream of images of any suitable modality.

The registration circuitry 24 is configured to perform a 2D/3D registration process comprising stages 44 to 54 of the flow chart of FIG. 5. In the 2D/3D registration process, the 3D data 40 is aligned with the live image 42.

At stage 44, an initial registration transform is applied to the 3D data set 40 to select a plane. The selected plane in the 3D data may correspond to a plane of the live image 42.

The initial registration transform that is used may be based on, for example, a registration transform obtained from registration of a previous frame of a stream of frames. In some circumstances (for example, when the 2D/3D registration is performed on the first frame of the series), the initial registration transform may be obtained using a known relationship between the scanning geometries of the CT scanner 14 and the X-ray scanner 15. In some embodiment, the initial registration transform may be obtained from a manual alignment process in which a user manually aligns a 2D image with the 3D data.

In the present embodiment, the initial registration transform is a rigid transform. In other embodiments, the initial registration transform may be any suitable rigid or non-rigid transform. The initial registration transform may comprise rotation, translation and/or scaling parameters. The initial registration transform may comprise deformation parameters, for example warp parameters.

At stage 46, the rendering circuitry 28 renders a Digitally Reconstructed Radiograph (DRR) from the 3D data set. The initial registration transform is used to select a plane of the DRR.

The DRR is a 2D projection of the 3D data set 40. Springer's Encyclopedia of Radiation Oncology describes a Digitally Reconstructed Radiograph as ‘A reconstructed image or set of images in planes other than the original plane of image acquisition that are generated by a computer system’. To form the DRR, a projection algorithm projects data of the 3D data set onto the plane that is not the original plane of data acquisition.

The plane on which the data is projected may be oriented in any way in space. In the present embodiment, the plane is determined using the initial registration transform.

In the present embodiment, the projection algorithm uses Average Intensity Projection to obtain the pixel values of the DRR. In other embodiments, the projection algorithm may use any one of a variety of projection methods to obtain the final individual pixel values of the DRR.

At stage 48, the registration circuitry 24 performs a 2D/2D registration. The 2D/2D registration comprises registering the DRR that was generated at stage 46 with the 2D image 42.

The 2D/2D registration may be performed using any suitable rigid or non-rigid registration method. In the present embodiment, the 2D/2D registration method comprises a multi-modality registration method.

The registration circuitry 24 calculates a value for a similarity measure, which in the present embodiment is mutual information.

At stage 50, the registration circuitry 24 assesses whether the 2D/2D registration is improving by comparing the value for the similarity measure obtained at stage 48 to at least one previous value for the similarity measure.

In the first instance of stage 48, there is no previous value for the similarity measure to which to compare, so the process proceeds to stage 52.

At stage 52, the registration circuitry 24 generates an updated registration transform. In the present embodiment, the registration circuitry generates the updated registration transform by changing a value for at least one of a rotation parameter, a translation parameter and a scaling parameter when compared with the previous registration transform. (In the first instance of stage 52, the previous registration transform is the initial registration transform.) The parameters may be changed using any appropriate optimization method, for example Powell's algorithm.

The process proceeds to a second instance of stage 44. At stage 44, the updated registration transform is applied to the 3D data set 40 to obtain a further plane.

At stage 46, the rendering circuitry 28 renders a new DRR from the 3D data set 40 by projecting the 3D data set on the further plane. The new DRR is projected on a different plane from the first DRR, because the registration transforms used are different.

At stage 48, the registration circuitry 24 registers the new DRR to the 2D image 42 and outputs a new value for the similarity measure.

At stage 50, the registration circuitry 24 determines whether registration is improving by comparing the new value for the similarity measure to the previous value for the similarity measure. In the present embodiment, the similarity measure is mutual information. The registration circuitry determines that the registration is improving if the new value for mutual information is higher than the previous value. In other embodiments, the similarity measure may be such that a lower value represents an improved registration.

In the present embodiment, the registration circuitry 24 determines whether registration is improving by comparing the new value for the similarity measure to the immediately preceding value for the similarity measure. In other embodiments, the registration circuitry may compare the new value for the similarity measure to any suitable number of previous values, for example the values for the similarity measure that were calculated in the preceding 2, 3 or 4 instances of stage 48.

If the registration circuitry 24 determines that the registration is improving, the process returns once more to stage 52 and the registration transform is again updated as described above.

If the registration is no longer improving, the process of FIG. 5 moves to stage 54.

At stage 54, the 2D/3D registration process of FIG. 5 ends. The registration circuitry 24 outputs a final transform. The final transform is a 3D transform that transforms the coordinate system of the 3D data set 40 into the coordinate system of the 2D image 42.

The final transform may be obtained by combining the final updated registration transform (which is a 3D transform) with a 2D transform obtained in the final 2D/2D registration of stage 48.

The final transform may be the transform resulting in the best (for example, highest) value of the similarity measure.

The 2D/3D registration process of FIG. 5 relates the coordinate space of the 3D data to the coordinate space of a selected frame of the live stream of 2D images.

The final transform may be used to select a rendering direction from which to render an image from the 3D data, such that the resulting rendered image is in the same coordinate space as the 2D image. The rendering direction may be a direction defining a rendering plane, for example a on which a slice is taken, or a plane on which an image is projected. The rendering direction may be a viewing direction. The rendering direction may be defined in relation to a viewpoint.

One method of implementing the 2D/3D registration process of FIG. 5 could be to repeat the 2D/3D registration process for each frame of the live stream of 2D/3D images. However, it has been found that a time taken to complete the 2D/3D registration may be longer than an inter-frame interval.

If the 2D/3D registration process were to be performed for every one of the 2D images, it may not be able to keep up with the rate at which the 2D images are produced. The 2D/3D registration may be time-intensive and/or have high computational requirements.

Therefore, in a method described below with reference to FIG. 6 and FIG. 7, 2D/3D registration is performed only for selected frames of a stream, and not for every frame. A decision of whether to perform a 2D/3D registration for a given frame is made based on whether a condition is satisfied. In the present embodiment, determining whether the condition is satisfied comprises determining whether an estimated misalignment for the frame exceeds a first threshold value and determining whether a time elapsed since a last 2D/3D registration exceeds a second threshold value.

FIG. 6 is a flow chart illustrating in overview the process of an embodiment. In the process of FIG. 6, a 2D/3D registration process is performed for only some 2D images in a stream of 2D images. Other images of the stream of 2D images undergo a 2D/2D registration to a previous image of the stream of 2D images.

When a new 2D/3D registration is performed on one of the 2D images, it is performed in parallel with the acquisition of further 2D images. By performing the new 2D/3D registration in parallel with the acquisition of further 2D images, a delay in the processing and display of live images may be avoided.

Stage 60 of FIG. 6 is an initialization stage. At stage 60, the assessment circuitry 26 receives a 3D transform 76 from the registration circuitry 24. The 3D transform has been obtained from a 2D/3D registration process 74.

The assessment circuitry 26 further receives a 2D image, which may be referred to as a 2D reference image. The 2D reference image is one frame of the live stream of fluoroscopy frames acquired by the X-ray scanner 15. The 2D reference image is the image with which the 3D data was registered in the 2D/3D registration process 74 to obtain the 3D transform 76.

The assessment circuitry 26 performs an error simulation process using the 2D reference image. The output of the error simulation process is a first threshold value, which in the present embodiment is a threshold value for a similarity measure. The first threshold value is subsequently used in stage 64 of the process of FIG. 6 as described below.

The error simulation process of the present embodiment is described below. In other embodiments, a different error simulation process may be used.

In the present embodiment, the assessment circuitry 26 stores the threshold for the similarity metric. The threshold for the similarity metric is based on a predetermined Target Registration Error.

The Target Registration Error is a distance between the position of at least one target in a first image and the at same at least one target in a second image, when the first image and second image are registered to each other. The first image may be referred to as a registered image and the second image may be referred to as a ground truth image. The at least one target may comprise at least one region of interest and/or at least one landmark. The at least one target may comprise at least one anatomical feature and/or at least one anatomical landmark.

In the present embodiment, the Target Registration Error is 1 mm. A registration may be considered to be acceptable if position of one or more targets of interest differs by less than 1 mm between the first image and the second image.

The error simulation process simulates multiple instances of misalignment, which in the present embodiment comprise misalignments by rotation and misalignments by translation. Each misalignment is simulated by offsetting a copy of the reference image such as to provide a Target Registration Error of 1 mm. Rigid transforms are obtained by sampling values of rotation and translation that result in a TRE of 1 mm. It may be considered that we map a parameter space around an area corresponding to 1 mm TRE.

The assessment circuitry 26 then computes a registration cost function for each offset copy of the reference image and the original reference image.

One example of a simulated misalignment is now described with reference to FIG. 7.

FIG. 7 is a schematic illustration of a reference image 78 comprising a region of interest around an anatomical feature. In the example of FIG. 7, the region of interest is around a vertebra 80. A region of interest diameter (ROI diameter) 82 of the region of interest is defined such that the region of interest contains the anatomical feature. In other embodiments, the region of interest may be defined in any suitable manner. The region of interest may or may not be defined with reference to a particular anatomical feature.

The Target Registration Error 84 for a rotation of the region of interest 82 is illustrated in FIG. 7.

In one instance of misalignment, a copy of the reference image 78 is rotated such that the largest displacement of any point within the region of interest having ROI diameter 82 equals the value of the Target Registration Error, for example 1 mm. In the present example, the reference image 78 is rotated by the angle 85 illustrated in FIG. 7.

A value for a registration cost function is calculated for the rotated copy of the reference image 78 and the original reference image 78. In the present embodiment, the registration cost function is a similarity metric. The similarity metric may be any similarity metric that is appropriate for representing the similarity between two 2D images of the same modality. In the present embodiment, the registration cost function is representative of a Euclidean distance between features of the rotated copy and the original reference image. The registration cost function may also be referred to as an index value.

In the present embodiment, a low value of the registration cost function will result from images that are very similar to each other, and a high value of the registration cost function will result from images that are dissimilar.

The error simulation process comprises multiple instances of misalignment. Some of the misalignments comprise rotation and some of the misalignments comprise translation.

A value for the registration cost function is determines for each of the misalignments. The assessment circuitry 26 then selects the lowest value of the registration cost function that is obtained for any of the misalignments. The lowest value may also be referred to as a minimum value. The assessment circuitry 26 sets the lowest value of the registration cost function as a first threshold value.

The value of the registration cost function may be highly dependent on the content of the image and/or the image properties of the image. Therefore, the first threshold value that is determined by the error simulation process may differ considerably from image to image, even where the same Target Registration Error is used.

The first threshold value sets a value for a similarity metric that is considered to be representative of an acceptable amount of misalignment.

The process of obtaining the threshold value is referred to as an error simulation process because it uses an artificially induced misalignment between two identical images.

The error simulation process is performed in a single plane, which is the plane of the 2D reference image. Misalignment is simulated by simulating rotation and translation in the 2D plane of the 2D reference image. However, the first threshold value is later used at stage 64 to assess whether there is likely to have been an out-of-plane misalignment. The use of the first threshold value to assess whether there has been misalignment is discussed further below with reference to stage 64

At the end of stage 60, the assessment circuitry 36 outputs the first threshold value. The first threshold value is specific to the 2D reference image with which the current 3D transform 76 was obtained.

At stage 62, a new 2D image is passed to the registration circuitry 24. The new 2D image is a further frame of the live stream of fluoroscopy frames acquired by the X-ray scanner 15.

The registration circuitry 24 performs a 2D/2D registration process to register the new 2D image to the 2D reference image. Any suitable 2D/2D registration method may be used. For example, the 2D/2D registration method may comprise optical flow. The 2D/2D registration may comprise a classic optimization-based registration method, for example mutual information or cross-correlation paired with Powell's algorithm.

From the 2D/2D registration, the registration circuitry 24 obtains a value for the registration cost function. The registration cost function is the same registration cost function that was used at stage 60, which in the present embodiment is a least squares similarity metric. The value for the registration cost function may also be referred to as an index value.

The value for the registration cost function is representative of the similarity of the new 2D image and the 2D reference image once they have been registered by 2D/2D registration. If the new 2D image and the reference 2D image are capable of being well registered, the value for the registration cost function is likely to be low. If the new 2D image and the reference 2D image are not capable of being well registered, the value for the registration cost function is likely to be high.

At stage 64, the assessment circuitry 26 assesses whether the error in the 2D/2D registration is too big by comparing the value for the registration cost function that was obtained at stage 62 to the first threshold value that was calculated at stage 60.

If the value for the similarity metric is higher than the first threshold value, the method of FIG. 6 proceeds to an asynchronous reference updating stage 72 which is described in detail below. If the value for the similarity metric is lower than the first threshold value, the method of FIG. 6 proceeds to stage 66.

At stage 66, the assessment circuitry 26 determines a length of time that has elapsed since a 2D/3D registration was last performed. The length of time elapsed may be defined as a time interval (for example 100 ms) or as a number of frames (for example, 3 frames).

The assessment circuitry 26 compares the time elapsed since the last 2D/3D registration to a second threshold value, which may also be described as a time threshold value. The second threshold value may be of the order of a few seconds. In some embodiments, the second threshold value is described as a number of frames, for example 20 frames. In some embodiments, the second threshold value is modifiable by the user. The second threshold value may depend on the patient. The second threshold value may depend on scanning conditions and/or intervention type.

If the value for the time elapsed is higher than the time threshold value, the method of FIG. 6 proceeds to the asynchronous reference updating stage 72. If the value for the time elapsed is lower than the time threshold value, the method of FIG. 6 proceeds to stage 68.

At stage 68, the rendering circuitry 28 renders an image from the 3D data set. The rendering circuitry 28 selects a rendering direction from which to render the image based on the 3D transform obtained at stage 60. The rendering direction may also be based on a result of the 2D/2D registration of stage 62.

In the present embodiment, the rendered image is a two-dimensional projection of the 3D data set. In other embodiments, the rendered image is a volume rendering, using appropriate rendering parameters. For example, in the case of a spine intervention, an image rendered from the 3D data set may focus on high brightness materials such as bone. In other cases, the rendering may focus on the soft tissue. In some embodiments, the image rendered may show contrast-enhanced blood. In further embodiments, the rendered image is a DRR.

The rendered image is displayed to a user on the display screen 16. The 2D image is also displayed to the user on the display screen 16. Further images may also be displayed. Images may be displayed in accordance with a hanging protocol which specifies a configuration in which multiple images are to be displayed on a screen or screen.

By using the 3D transform obtained from the 2D/3D registration to obtain the rendering direction, the image rendered from the 3D data set is aligned with the new 2D image. A user viewing the rendered image and the new 2D image may see corresponding anatomical features displayed at common locations in the two images.

In further embodiments, the rendered image comprises a fusion image in which data from the 3D data set is fused with data from the new 2D image.

After the rendering and display of the rendered image at stage 68, the method returns to stage 62, at which a next 2D image 70 (which may also be referred to as a next frame) is received by the registration circuitry 24. The next 2D image is a further live 2D fluoroscopy image acquired by the X-ray scanner 15.

At stage 62, the next 2D image 70 is registered to the 2D reference image. From the 2D/2D registration, the registration circuitry 24 obtains a value for the registration cost function for the next 2D image 70.

At stage 64, the assessment circuitry 26 determines whether the value for the registration cost function is too large. At stage 66, the assessment circuitry 26 determines whether a time elapsed since last 2D/3D registration is too long. At stage 68, an image is rendered from the 3D data and is displayed along with the next 2D image 70.

Stages 62 to 68 are repeated until a condition for triggering a 2D/3D registration is satisfied. In the present embodiment, the condition has two parts. The condition is satisfied if the first part and/or the second part of the condition is satisfied. The first part of the condition is that the value for the registration cost function for a given frame exceeds the first threshold value. The second part of the condition is that the time since the last 2D/3D registration exceeds the second threshold value. Therefore, stages 62 to 68 are repeated as long as the error does not get too high, and not too much time has elapsed.

We now consider what happens if the condition is satisfied. If the condition is satisfied for a 2D image, a new 2D/3D registration is performed in which that 2D image is registered with the 3D data. The new 2D/3D registration is performed in parallel with the acquisition of further 2D images.

We consider an instance of stage 64 in which the assessment circuitry 26 determines that the value for the registration cost function for the present 2D image exceeds the first threshold value. The registration circuitry 24 selects the present 2D image for 2D/3D registration. The present 2D image may be referred to as a selected 2D image.

In response to the determining that the value for the registration cost function for the present 2D image exceeds the first threshold value, the method of FIG. 6 triggers an asynchronous reference updating stage 72.

We note that the repeating operation of stages 62 to 68 is not paused while the asynchronous reference updating stage 72 is performed. Instead, the repetition of stages 62 to 68 for each newly acquired 2D image of the stream of images proceeds in accordance with the times at which those images are received. The asynchronous reference updating stage 72 is performed in parallel. This timing and parallel operation is discussed further below with reference to FIG. 8.

The asynchronous reference updating stage 72 comprises a 2D/3D registration 74. At stage 74, the registration circuitry 24 performs a 2D/3D registration of the selected 2D image with the 3D data set. The method used by the registration circuitry 24 to perform 2D/3D registration is as described above with reference to FIG. 5. The registration circuitry 24 repeatedly generates a DRR from the 3D data set and registers the DRR with the selected 2D image using a multi-modality registration method. In other embodiments, any suitable 2D/3D registration method may be used.

The 2D/3D registration may use as an initial registration transform the 3D transform obtained from the previous 2D/3D registration. The 2D/3D registration may use as an initial registration transform a version of the 3D transform that has been transformed in 2D in accordance with the 2D/2D registration of the selected 2D image with the reference 2D image.

The output 76 of the 2D/3D registration 74 is an updated 3D transform that transforms the coordinate system of the 3D data set into the coordinate system of the selected 2D image.

The updated 3D transform 76 is provided to the assessment circuitry 26. The assessment circuitry 26 performs an initialization stage 60 using the updated 3D transform and the selected 2D image. The initialization stage 60 comprises establishing an updated value for the first threshold value using a method as described above.

The assessment circuitry 26 uses the selected 2D image as a new reference image. The method proceeds again to stage 62, at which a 2D/2D registration is performed to register a current 2D image and the new reference 2D image. The processes of stages 62 to 68 are performed on subsequent images as described above, using the new reference 2D image and the updated 3D transform.

In FIG. 6, dotted lines are used to indicate that the reference updating stage 72 is asynchronous with the stream of 2D images. While the selected 2D image is being registered with the 3D data in the reference updating stage 72, one or more subsequent 2D images 70 are acquired and are registered to the original reference 2D image at stage 62, and have images rendered for display at stage 68.

This asynchronous 2D/3D registration process is further illustrated in the schematic diagram of FIG. 8.

FIG. 8 illustrates a stream of 2D images 80A to 80G. Time is represented by a horizontal axis 82. The positioning of each of the stream of 2D images 80A to 80G along the time axis 82 indicates a time at which each of the 2D images is acquired and/or processed.

We consider the process described above with reference to FIG. 6, from the point of view of successive frames of the stream of frames.

The first frame 80A is used as a 2D reference image, and is denoted as R.

At stage 62, a second frame 80B is registered to the first frame 80A by performing a 2D/2D registration which is indicated by arrow 84A. At stage 64, the 80B is not found to have an excessive error. At stage 66, it is found that not too much time has elapsed. Therefore, no further registration is triggered by the second frame 80B. An image is rendered from the 3D data at stage 68, and the process of FIG. 6 returns to stage 62.

At stage 62, the third frame 80C is registered to the first frame 80A by performing a 2D/2D registration which is indicated by arrow 84B. In the example shown in FIG. 8, it is found at stage 64 that the error in the 2D/2D registration is above the first threshold value.

An asynchronous reference updating stage 72 is triggered. The triggering of the asynchronous reference updating stage 72 is indicated by arrow 86. The asynchronous reference updating stage 72 is described below.

In parallel with the reference updating stage 72, the process proceeds to stage 66 for the third frame, at which it is found that the time elapsed is not above the second threshold value. In some embodiments, stage 66 may be omitted if the error at stage 64 is found to be above the first threshold value. An image is rendered from the 3D data at stage 68.

A first vertical line 88 is used to indicate the time at which the asynchronous reference updating stage 72 starts. The asynchronous reference updating stage 72 comprises a 2D/3D registration as described above with reference to FIGS. 5 and 6. The 2D/3D registration may also be describes as a DRR registration or DRR-based registration. In the 2D/3D registration, a plurality of DRRs are generated from the 3D data and are registered to the third frame 80C until convergence is reached.

Stage 90 is the start of the 2D/3D registration (also referred to as DRR-based registration). Stage 92 is the end of the 2D/3D registration.

The DRR-based registration computes a DRR 94 that best matches the reference fluoroscopy, which in this case is the third frame 80C. We note that the positioning of the DRR 94 in FIG. 8 does not reflect a time at which the DRR 94 is generated, but is instead used to indicate proximity to the third frame 80C. An output of asynchronous reference updating stage 72 is an updated transform 96 which relates the coordinate system of the 3D data to the coordinate system of the third frame 80C.

A second vertical line 98 is used to indicate the time at which the 2D/3D registration of asynchronous reference updating stage 72 has been completed.

Once the 2D/3D registration has been completed, the projection transform 96 that was used to create the DRR 94 is used as the initialization for future 2D/2D fluoroscopy registrations. An initialization stage 60 is performed for the updated transform 96 to obtain a new first threshold value, and the assessment circuitry 26 sets the third frame 80C as a new reference image R′.

It may be seen that the 2D/3D registration is not finished until after the fourth frame 80D has been acquired.

When the fourth frame 80D is acquired, the asynchronous reference updating stage 72 has not yet been completed. At stage 62, the fourth frame 80D is registered to the first frame 80A by performing a 2D/2D registration which is indicated by arrow 84C. At stage 64, the assessment circuitry 26 determines whether the error value for the fourth frame 80D exceeds the first threshold value. It may be the case that the error value for the fourth frame 80D does exceed the first threshold value, given that error value for the third frame 80D exceeded the first threshold value. However, the assessment circuitry 26 may prevent a further 2D/3D registration being performed for the fourth frame 80D if a 2D/3D registration is already in progress for the third frame 80C. In some embodiments, stage 64 may be omitted for a given frame if it is known that a previous frame that was registered to the same reference image has already triggered a 2D/3D registration.

At stage 66 for the fourth frame, it may be determined that the second threshold value has not been exceeded. In some embodiments, stage 66 may be omitted. At stage 68, an image is rendered from the 2D data using the original 3D transform (the same 3D transform that was used for the first image 80A, second image 80B and third image 80C). The original 3D transform is used since no new 3D transform is yet available. While the fourth frame 80D is being processed, the asynchronous 2D/3D registration of the third frame 80C has not yet been completed and the updated transform 96 has not yet been generated.

We turn to the fifth frame 80E. The fifth frame 80E is received after the asynchronous reference updating stage 72 has finished. Therefore, the updated transform 96 is now available. The third frame 80C is used as a new reference image R′.

At stage 62, registration circuitry 24 registers the fifth frame 80E to the third frame 80C by performing a 2D/2D registration which is indicated by arrow 100A. At stage 64, the assessment circuitry 26 determines whether the value for the registration cost function for the registration of the fifth frame 80E with the third frame 80C exceeds the updated first threshold value. At stage 66, the assessment circuitry 26 determines whether the time since the last 2D/3D registration (which was the asynchronous registration of the third frame 80C) exceeds the second threshold value. In the example shown, neither threshold is exceeded. At stage 68, the rendering circuitry 28 renders an image from the 3D data using the updated transform 96.

In the example shown, the registration circuitry 24 registers the sixth frame 80F to the third frame 80C by performing a 2D/2D registration which is indicated by arrow 100B. The registration circuitry 24 registers the seventh frame 80G to the third frame 80C by performing a 2D/2D registration which is indicated by arrow 100C. Neither the sixth frame 80F nor seventh frame 80G is found to have an excessive error, and not too much time has elapsed. Therefore, no further registration is triggered by the fifth frame 80E, sixth frame 80F or seventh frame 80G.

The alignment of the 3D data with the stream of 2D images may be used to provide navigation information to a clinician performing a needle intervention.

In the process of FIG. 6 and FIG. 8, a stream of intra-operative 2D fluoroscopy images are registered with pre-operative 3D image data to provide navigational information to a user. A 2D/3D registration is performed for only some of the fluoroscopy frames of the stream. Other fluoroscopy frames are registered to a well-aligned 2D image using 2D/2D registration.

Fluoroscopy-based live guidance of a needle intervention is performed using a mixture of 2D/3D and 2D/2D registrations. By combining 2D/3D and 2D/2D registration, a complexity of the method and/or its computational load may be reduced when compared with a method that uses only 2D/3D registrations.

In the method of FIG. 6 and FIG. 8, each new fluoroscopy frame is registered to a well-aligned 2D image. The expensive 2D/3D registration is computed much less frequently and may be considered to act as a correction to the 2D/2D registration. The 2D/3D registration is triggered by either time elapsed since last correction, or detection of excessive registration error in the 2D/2D registration.

As has been described above, a 2D/3D registration may be time-intensive and/or computationally complex. Therefore, in the method of FIG. 6 and FIG. 8, a 2D/3D registration is performed only when it is triggered by excessive error and/or a time elapsed since the previous 2D/3D registration.

If 2D/3D registration were to be performed for every frame, a high frame rate would require fast registration. In the method of FIG. 6 and FIG. 8, the expensive DRR-based registration is computed less frequently, and out of synch with the live fluoroscopy.

By performing 2D/3D registration for some, but not all, of the frames, a computational load may be decreased. A decrease in computational load may be useful where computational power is limited, for example where computational resources are shared. In some circumstances, reducing the number of 2D/3D registrations performed may allow more accurate 2D/3D registrations and/or 2D/2D registrations to be performed with available resources.

Much of the registration is computed as 2D/2D. The 2D/2D registration can be used to correct in plane movement, for example X or Y translation, rotations around the X axis, or scale changes due to movements in the X direction.

Each new fluoroscopy image is registered to a well-aligned 2D image (reference). The error in the 2D/2D registration is checked, based on value of the similarity metric. If the check fails, or if too much time has elapsed since previous resetting, a new 2D/3D DRR-based registration is launched as a background process. While the 2D/3D registration is performed, the 2D/2D registration is computed with each new frame. When the 2D/3D registration has completed, the resulting transform is applied to the 2D frame that was registered. That frame becomes the new reference image. The 2D/3D registration process may be referred to as asynchronous reference updating, as the initial transform for the 2D/2D reference image is updated whenever a new transform is available. There is no pause in the flow of new transforms to the application.

Out of plane rotations may be corrected with 2D/3D registration, which happens less frequently than 2D/2D registration.

By performing the 2D/3D registration of the third frame 80C in parallel with the acquisition and processing of further frame 80D, the 2D/3D registration may be performed without slowing the stream of images displayed to the user. Live fluoroscopy is maintained on display at all times.

The 2D/3D registration method may occupy a period of time that is longer than an inter-frame interval. In some circumstances, the use of parallel registration may provide more efficient use of computational resources. In some circumstances, the use of parallel registration may allow a more accurate 2D/3D registration to be performed than may be the case if the time allowed for 2D/3D registration was restricted.

In embodiments describes above, DRRs are rendered only when a 2D/3D registration is triggered. A 2D/3D registration is triggered only for selected frames of a stream of frames. Rendering a DRR can be time-intensive, especially if repeated at every step of an iterative registration process. By rendering DRRs only for a limited number of frames, computing resources used may be reduced. It may be possible to reduce or minimize a number of DRRs that are computed.

Live imaging obtained during a procedure may be obtained using a fixed imaging plane. However, it may still be the case that a 2D/3D registration does not remain well-aligned throughout an imaging procedure. For example, the subject may move relative to the imaging plane. An anatomical feature of the subject may be distorted. For example, the presence of the needle (or other medical device) may distort the subject's anatomy. Therefore, a further 2D/3D registration may be used to correct an existing 2D/3D registration.

The first threshold value is calculated by simulating an in-plane misalignment. The simulation may comprise taking a good registration and deliberately offsetting it in rotation to see how a similarity score degrades.

The simulated in-plane misalignment is then used as a proxy for an out-of-plane misalignment. The value for the similarity metric in the 2D/2D registration is used as a substitute for a registration error. It is assumed that a similarity space locally has a similar slope in all directions (in three dimensions). It is then assumed that an in plane rotation (as simulated) has a similar effect on similarity score as an out of plane rotation (not simulated). A simulated in plane rotation is used to simulate the effect on similarity score of an out of plane rotation. Registration error checking assumes that the cost function will be similar for in plane and out of plane rotation. There is no simulation of out of plane rotations with 2D images.

The value for the registration cost function that is obtained at stage 62 may be indicative of how good a 2D/2D registration between the current image and the reference image has been achieved. If the value for the registration cost function is low, a good registration has been achieved. If the value for the registration cost function is high, it may be the case that a good 2D/2D registration cannot be achieved. One reason for a good 2D/2D registration not being obtained may be that the current image is being obtained on a somewhat different 2D plane from that of the reference image. If this is the case, a 2D/3D registration may be used to align the 3D data with subsequent images.

In the embodiments described above, a first threshold value is used to determine whether a new 2D/3D registration is due. The first threshold value may be set conservatively. The first threshold value may be set such that a 2D/3D registration is triggered before excessive misalignment has occurred. The first threshold value may be set such that continuing to use an original 3D transform and reference image for a further number of frames (for example, one, two or three frame) while the new 2D/3D registration is performed may not result in excessive misalignment.

In the embodiments described above, each new 2D image is registered to a previous one of the 2D images, which is used as a reference image. In further embodiments, a DRR may be used as a reference image. Each of the 2D images may be registered to a DRR that was generated in a preceding 2D/3D image. Any suitable 2D/2D registration method may be used to register the 2D image with the DRR.

In embodiments described above, 2D/3D registration is triggered by either time elapsed since previous resetting, or by checking for excessive error in the 2D/2D registration. In other embodiments, any suitable condition may be used to trigger 2D/3D registration. In some embodiments, 2D/3D registration is triggered by a significant change in hardware conditions, for example a change is dose, exposure, field of view, collimation, or filtering method.

In some embodiments, 2D/3D registration is always triggered by time elapsed and not based on an error. For example, a 2D/3D registration may be performed for every second, third, fourth or fifth frame. In some embodiments, 2D/3D registration is triggered only by excessive error.

In some embodiments, 2D/3D registration is manually triggered by a user. A user may choose to perform 2D/3D registration at any arbitrary timing. For example, a user may choose to perform 2D/3D registration if the user suspects that there may be a misalignment. In some embodiments, a switch is provided to the user. The switch may be provided in hardware and/or in software. When the user switches the switch, a new 2D/3D registration is triggered.

In some embodiments, a user may terminate a 2D/3D registration, for example if the user suspects a misalignment. In some embodiments, if a user terminates a 2D/3D registration, the system ceases to display an image rendered from the 3D data until a new 2D/3D registration has been performed.

In some embodiments, the registration circuitry 24 is configured to terminate processing of automatic 2D/3D registration when the automatic processing fails continuously. User interfaces may return to a previous state when a 2D/3D registration fails. For example, if a registration fails, the system may return to a previous situation when the images were well registered. Alternatively, if a registration fails, the registration procedure may restart from the start, at which an initial registration needs to be provided.

In embodiments described above, the 2D/3D registration is performed in the background, and the user is not informed that a 2D/3D registration is being performed. In other embodiments, users may want to know that a present roadmap image rendered from the 3D data is not consistent with the real-time fluoroscopic image due to patient motion. The system may inform users that a misalignment has been detected (for example, that an error has exceeded the first threshold value) while performing 2D/3D registration. Any suitable method may be used to inform the user. For example, a graphical indication may be displayed on the display screen 17. A colour or warning may be displayed overlaid with the 2D image and/or the image that is rendered from the 3D data.

In other embodiments, an indicator of the misalignment is displayed at all times so that the user has information about the degree of misalignment, even if it is not high enough to trigger a 2D/3D registration.

A user interface may allow a user to change a misalignment threshold. For example, the user may change a Target Registration Error. A user may change an allowable distance between registered features.

A user interface may allow a user to change an allowable time between 2D/3D registrations, for example by changing the second threshold value.

In some embodiments, a user interface displays an indicator of the remaining time until 2D/3D registration is completed. In some embodiments, a user interface displays a time until a next scheduled 2D/3D registration, for example if 2D/3D registration is performed based on a time elapsed.

In embodiments described above, the live 2D images are displayed to a user (for example, a clinician) on display screen 16. A 2D image is rendered from the 3D data using a rendering direction that was determined by a 2D/3D registration. Any suitable rendering method may be used. Typically, the 2D/3D registration was performed on a previous 2D image of the stream of 2D images.

By displaying a 2D image that is rendered from the 3D data, additional information may be provided to the user, for example information obtained in a different modality. The user may be assisted in performing the procedure. For example, having more information about a 3D environment may assist navigation. In some cases, the procedure being performed may be a very sensitive one, for example injecting material into the spine.

In some embodiments, information from the 3D data is fused with the 2D image. For example, 3D CT data may be fused with live fluoroscopy data. Fusion images may be sequentially generated by fusing successive 2D images with an image rendered from the 3D data. When a new 2D/3D registration is performed, 2D images may be fused with a different image rendered from the 3D data, where the different image is rendered using a different rendering direction.

In some embodiments, image fusion is used to draw a position and/or path of a medical device (for example, a needle) onto an image rendered from the 3D data. Typically, the medical device was not present in the body when the 3D data was acquired. The position of the medical device may be obtained from the 2D images. In other embodiments, the position of the medical device may be obtained using any suitable method, for example using a position detector.

In some embodiments, image fusion is used to indicate a target location in the 2D image and/or in an image rendered from the 3D data.

Embodiments above are described with regard to a needle intervention. In other embodiments, methods described above may be used in any procedure in which pre-operative 3D data is registered with intra-operative 2D data. The procedure may comprise a catheter-based intervention or probe-based intervention. The procedure may comprise, for example, taking a biopsy sample or performing an ablation. The procedure may comprise a musculoskeletal intervention, for example vertebroplasty or a placement of a prosthetic device. The procedure may comprise a cardiac valve repair or replacement intervention. The procedure may comprise laparoscopy.

A medical device that is being guided within the body may comprise, for example, a needle, a stent, a replacement body part, a prosthetic, a measurement device, a surgical device, a valve, a heart valve, an imaging device, an endoscopic device, a catheter, an implant, or an electronic device (for example, an endoscope or an ultrasonic probe).

Methods described above may be applied to tumour ablation and embolization, aneurysm treatment, vascular interventions, or any other intervention where target guidance is needed. In the case of vascular interventions, special clinical protocols may be used to allow the visualisation of blood vessels.

The procedure may be any procedure for which live imaging is performed. In the embodiments above, the live imaging is fluoroscopy imaging. In other embodiments, the live imaging may comprise, for example, ultrasound imaging or video imaging.

Embodiments described above as being performed using the apparatus of FIG. 3 may also be performed using the apparatus of FIG. 4.

Certain embodiments provide a registration method comprising:

a) Acquiring a 3D volume of an anatomical area;

b) Acquiring a continuous stream of 2D images of the same anatomy;

c) Registering initial 2D image to 3D volume;

d) Registering each new 2D image to registered reference 2D image;

e) Checking the quality of the registration in step (d);

f) Performing a new 2D/3D registration between another 2D image acquired later and the 3D volume if the check in step (e) has failed

g) Updating the reference 2D image to the one registered in step (f);

h) Repeat step (d), using the result transform of step (f) for initialization; in which steps (f) and (g) are performed as a background process.

The 3D Volume may be CT, MR, or CBCT. The 2D images may be fluoroscopy, X-ray, or ultrasound. The 2D/3D registration may be performed using a DRR-based method.

If the check in step (e) is successful, a backup check on the time elapsed since last instance of step (f) has been performed may be used.

Step (f) may be performed as often as possible to maintain runtime of step (d) smaller than the time between each new frame coming from step (b), thus ensuring live alignment for guidance.

In step (d), the registration may be applied to the 3D volume to align it to the new 2D image.

Certain embodiments provide a medical image processing apparatus, comprising processing circuitry configured to: acquire at least one two-dimensional image, set a rendering direction based on three-dimensional image and the at least one two-dimensional image, generate a rendered image from the three-dimensional image, based on the rendering direction, analyze a misalignment between a newly generated two-dimensional image and the rendered image, correct the misalignment by processing on the rendered image, reset the rendering direction when a condition concerning the misalignment is satisfied, and regenerate the rendered image, based on the reset rendering direction.

The processing circuitry may be further configured to generate a fused image in which the rendered image is fused with the newly generated two-dimensional image.

The condition concerning the misalignment may be that an index value indicating a magnitude of the alignment between the newly generated two-dimensional image and the corrected rendered image exceeds a first threshold value.

The processing circuitry may be further configured to: sequentially generate, in parallel with processing resetting the rendering direction, the fused image by fusing, with the newly generated two-dimensional image, the rendered image generated based on the rendering direction before reset, sequentially generate, when finishing processing resetting the rendering direction, the fused image by fusing, with the newly generated two-dimensional image, the rendered image generated based on the rendering direction after reset.

The processing circuitry may be further configured to reset the rendering direction when an elapsed time since resetting the rendering direction last exceeds a second threshold value.

The three-dimensional image may be captured by an X-ray CT apparatus, an MRI apparatus or an X-ray diagnostic apparatus.

The at least one two-dimensional image and the newly generated two-dimensional image may be captured by an X-ray diagnostic apparatus or an ultrasonic diagnostic apparatus.

Certain embodiments provide an X-ray diagnostic apparatus, comprising: an X-ray tube configured to irradiate X-rays; an X-ray detector configured to detect X-rays irradiated from the X-ray tube; and processing circuitry configured to acquire at least one two-dimensional X-ray image based on a detection result by the X-ray detector, set a rendering direction based on three-dimensional image and the at least one two-dimensional X-ray image, generate a rendered image from the three-dimensional image, based on the rendering direction, analyze a misalignment between a newly generated two-dimensional X-ray image and the rendered image, correct the misalignment by processing on the rendered image, reset the rendering direction when a condition concerning the misalignment is satisfied, and regenerate the rendered image, based on the reset rendering direction.

Certain embodiments provide a medical image processing method, comprising: acquiring at least one two-dimensional image, setting a rendering direction based on three-dimensional image and the at least one two-dimensional image, generating a rendered image from the three-dimensional image, based on the rendering direction, analyzing a misalignment between a newly generated two-dimensional image and the rendered image, correcting the misalignment by processing on the rendered image, resetting the rendering direction when a condition concerning the misalignment is satisfied, and regenerating the rendered image, based on the reset rendering direction.

Embodiments have been described with regard to a needle intervention. In other embodiments, methods described above may be used for navigation of any suitable medical device. The medical device may be inserted into any part of any human or animal body. The medical device may be used in any suitable medical or veterinary procedure.

Whilst particular circuitries have been described herein, in alternative embodiments functionality of one or more of these circuitries can be provided by a single processing resource or other component, or functionality provided by a single circuitry can be provided by two or more processing resources or other components in combination. Reference to a single circuitry encompasses multiple components providing the functionality of that circuitry, whether or not such components are remote from one another, and reference to multiple circuitries encompasses a single component providing the functionality of those circuitries.

Whilst certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms and modifications as would fall within the scope of the invention. 

1. A medical image processing apparatus, comprising processing circuitry configured to: obtain a three-dimensional (3D) image that is representative of an anatomical region of a subject; acquire a stream of two-dimensional images that are representative of the anatomical region of the subject; set a first rendering direction by performing a 2D/3D registration procedure in respect of the 3D image and a 2D image of the stream of 2D images; generate a first rendered image from the 3D image based on the first rendering direction; for each of a plurality of subsequent 2D images of the stream of 2D images, determine whether a condition is satisfied, the condition being dependent on at least one of a 2D misalignment and a time since last 2D/3D registration procedure; and, when the registration condition is satisfied, select one of the subsequent 2D images; reset the rendering direction to obtain a second rendering direction by performing a 2D/3D registration procedure in respect of the 3D image and the selected 2D image; and regenerate a second rendered image from the 3D image based on the second rendering direction.
 2. An apparatus according to claim 1, wherein the 3D image comprises a pre-operative image obtained before performing a procedure on the subject, and the stream of 2D images comprises a stream of intra-operative 2D images acquired in real time or near-real time during the performing of the procedure on the subject.
 3. An apparatus according to claim 1, wherein the 2D/3D registration procedure is performed in parallel with the acquisition of further 2D images of the stream of 2D images.
 4. An apparatus according to claim 1, wherein for each of the subsequent 2D images, the processing circuitry is configured to perform a respective 2D/2D registration between the subsequent 2D image and the first 2D image, and to determine an index value indicating a magnitude of an alignment between the subsequent 2D image and the first 2D image.
 5. An apparatus according to claim 4, wherein determining whether the registration condition is satisfied comprises determining whether the index value exceeds a first threshold value.
 6. An apparatus according to claim 1, wherein the processing circuitry is further configured to generate a fused image in which the first rendered image is fused with at least one of the subsequent 2D rendered images.
 7. An apparatus according to claim 1, wherein the processing circuitry is further configured to: sequentially generate, in parallel with the 2D/3D registration of the 3D image and the selected 2D image, fused images by fusing further 2D images with the first rendered image; and sequentially generate, once the 2D/3D registration of the 2D image and the selected 2D image is complete, fused images by fusing further 2D images with the second rendered image.
 8. An apparatus according to claim 1, wherein the condition is satisfied when a time since the last 2D/3D registration procedure exceeds a second threshold value.
 9. An apparatus according to claim 5, wherein the processing circuitry is further configured to determine the first threshold value by: inducing at least one misalignment of the first 2D image; and for the or each misalignment, determining a respective index value; and determining the first threshold value based on the index value or values for the at least one induced misalignment.
 10. An apparatus according to claim 9, wherein the induced misalignment comprises at least one of a rotation, a translation, a scaling.
 11. An apparatus according to claim 1, wherein the processing circuitry is further configured to receive a user request for re-registration, and to perform a 2D/3D registration of the 3D image with one of the stream of 2D images in response to the user request.
 12. An apparatus according to claim 1, wherein the processing circuitry is further configured to provide at least one of a) to e):— a) a graphical indication that the condition is satisfied; b) a graphical indication of a progress of a 2D/3D registration procedure; c) a graphical indication of a time until next 2D/3D registration procedure; d) a graphical indication of an index value; e) a graphical indication of a first threshold value and/or second threshold value.
 13. An apparatus according to claim 1, wherein the procedure comprises at least one of: fluoroscopy-guided needle biopsy, tumor ablation, tumor embolization, treatment of aneurysm, vascular intervention, musculoskeletal intervention, cardiac valve repair, cardiac valve replacement, laparoscopy.
 14. An apparatus according to claim 1, wherein the 3D image is captured by at least one of an X-ray CT apparatus, an MRI apparatus, an X-ray diagnostic apparatus.
 15. An apparatus according to claim 1, wherein the stream of 2D images is captured by an X-ray diagnostic apparatus or an ultrasonic diagnostic apparatus.
 16. An X-ray diagnostic apparatus medical image processing apparatus, comprising: an X-ray tube configured to emit X-rays; an X-ray detector configured to detect X-rays emitted by the X-ray tube; and processing circuitry configured to: obtain a three-dimensional (3D) image that is representative of an anatomical region of a subject; acquire a stream of two-dimensional images that are representative of the anatomical region of the subject; set a first rendering direction by performing a 2D/3D registration procedure in respect of the 3D image with a 2D image of the stream of 2D images; generate a first rendered image from the 3D image based on the first rendering direction; for each of a plurality of subsequent 2D images of the stream of 2D images, determine whether a condition is satisfied, the condition being dependent on at least one of a 2D misalignment and a time since last 2D/3D registration procedure; and, when the registration condition is satisfied, select one of the subsequent 2D images; reset the rendering direction to obtain a second rendering direction by performing a 2D/3D registration procedure in respect of the 3D image and the selected 2D image; and regenerate a second rendered image from the 3D image based on the second rendering direction.
 17. A medical image processing method, comprising: obtaining a three-dimensional (3D) image that is representative of an anatomical region of a subject; acquiring a stream of two-dimensional images that are representative of the anatomical region of the subject; setting a first rendering direction by performing a 2D/3D registration procedure in respect of the 3D image and a 2D image of the stream of 2D images; generating a first rendered image from the 3D image based on the first rendering direction; for each of a plurality of subsequent 2D images of the stream of 2D images, determining whether a condition is satisfied, the condition being dependent on at least one of a 2D misalignment and a time since last 2D/3D registration procedure; and, when the condition is satisfied: selecting one of the subsequent 2D images; resetting the rendering direction to obtain a second rendering direction by performing a 2D/3D registration procedure in view of the 3D image and the selected 2D image; and regenerating a second rendered image from the 3D image based on the second rendering direction. 